Composition for the storage of hydrogen and method of making the composition

ABSTRACT

A magnesium powder composition for the reversible storage of hydrogen by absorption thereof in the magnesium to form the hydride which consists of an intimate mixture of magnesium powder and an inert powder which is stable under the conditions of formation and decomposition of the hydride. The stable inert powder, which prevents the agglomeration of the magnesium powder and maintains its hydrogen-storage capacity, is selected from the group which consists of metal powders such as manganese, iron, cobalt, nickel and copper, metal alloy powders such as iron-zinc alloys, and the oxides, carbides or nitrides of magnesium, calcium, boron, aluminum, silicon, titanium, zinc, vanadium, chromium, manganese and iron. The mixture is formed by intimately grinding the magnesium and inert powders together.

FIELD OF THE INVENTION

The present invention relates to the storage of hydrogen as aninterstitial and/or stoichiometric hydride of magnesium and, moreparticularly, to an improved magnesium-based composition for the storageof hydrogen and to a method of making this composition and a method forstoring hydrogen utilizing this composition.

BACKGROUND OF THE INVENTION

In recent years, the storage of hydrogen as a potential fuel orreactant, has become of increasing interest and numerous systems havebeen described whereby hydrogen can be stored as an interstitial hydrideor stoichiometric compound of an appropriate metal, to be released asrequired, the storage systems being reversible.

One of these systems utilizes magnesium (Mg) which can form the hydride(MgH₂) from which hydrogen can be driven in gaseous form. A storagesystem based upon the reversible reaction H₂ +Mg=MgH₂ is thus capable ofstoring hydrogen from a gaseous state upon contact of the hydrogen withthe metal and of releasing hydrogen in a gaseous form at a subsequenttime and, if desired, at a different place.

While a number of other materials have also been proposed for thestorage of hydrogen in the form of respective hydrides, magnesium hasbeen found to be of interest because of its relatively low cost andlight weight which allows for a theoretical capacity of 7.6% by weightof hydrogen (based upon the weight of metal) to be stored andregenerated.

The storage of hydrogen in the form of magnesium hydride is described,for example, in French Pat. No. 1,529,371 and British Pat. No.1,171,364.

The use of the Mg/MgH₂ system for the reversible storage of hydrogen onan industrial scale, however, poses several practical problems.

For example, the magnesium should be in the form of a powder so as toobtain the maximum specific surface area for hydrogen absorption andhence for the conversion of the Mg to MgH₂ under acceptable conditions.

Magnesium powder produced by thermal decomposition has a conversionratio of MgH₂ /Mg greater than 0.9. The hydride can be producedinitially by permitting the magnesium to absorb hydrogen at atemperature of 327° C. at a pressure of 2.3 bar for a period of 6 hours.However the cost of the MgH₂ prepared by indirect techniques is too highto permit economical use of the magnesium powder thus obtained for thestorage of hydrogen in the form of a hydride by most economic criteria.

Furthermore, efforts to use magnesium turnings, instead of magnesiumpowder, for the storage of hydrogen have required conditions so extremeas to render the system impractical notwithstanding the lower cost ofthe starting material. For example, fine magnesium turnings must bemaintained for several days at a temperature of 400° to 450° C. at apressure of 70 bar in a hydrogen atmosphere for a molar conversion ratioMgH₂ /Mg of 0.9. These absorption conditions cannot be readily realizedwithout special equipment and make the storage of hydrogen on anindustrial scale impractical wherever these conditions must be observed.

Finally it is a practical necessity to provide conditions which can begenerated conveniently and economically for absorption and desorption ofhydrogen and yet provide a conversion ratio of MgH₂ /Mg which is as highas possible to obtain best utilization of magnesium. In other words thismolar ratio should be as high as possible, the duration required forabsorption and desorption should be as low as possible and both theabsorption temperature and absorption pressure should be kept as low aspossible.

It has been proposed to reduce the absorption temperature, theabsorption pressure and the duration required for complete hydrogenationof magnesium by providing the magnesium in the form of an alloy withcopper or nickel, namely, intermetallic compounds such as Mg₂ Cu and Mg₂Ni. The advantages of these materials is that they allow practicallycomplete transformation of magnesium to MgH₂ at a temperature of 200° C.and 300° C. with a hydrogen pressure of 15 bar.

The state of the art relating to the storage of hydrogen in magnesiumalloys is illustrated by the following works and publications:

D. L. Douglas: The Storage and Release of Hydrogen From Magnesium AlloyHydrides for Vehicular Applications, International Symposium on Hydridesfor Energy Storage, Norway, August 1977;

"Preparation of Magnesium Hydride", Russian Journal of InorganicChemistry, pp. 389-395, April 1961;

"The Reaction of Hydrogen with Alloys of Magnesium and Copper",Inorganic Chemistry, Vol. 6, No. 12, December 1967; and

"The Reaction of Hydrogen with Alloys of Magnesium and Nickel and theFormation of Mg₂ NiH₄ ", Inorganic Chemistry, Vol. 7, No. 11, November1967.

A disadvantage of magnesium alloys for the purposes described is thatthe magnesium alloys are comparatively costly since they must beprepared by smelting the elements, casting the resulting melt,comminuting the cast body and milling the comminuted product to a finepowder capable of absorbing hydrogen rapidly.

It has also been proposed to provide catalysts for an increase in thereaction rate of hydrogen with the magnesium powder. Such catalysts canbe organic compounds, generally organohalides or metals or alloys,especially titanium, vanadium, LaNi₅ and TiFe, which are known to reactreadily with hydrogen to form respective hydrides as is described inFrench application No. 75 28 647.

Such catalysts are additives whose use may be incompatible withindustrial exploitation of magnesium-based hydrogen storage systems forindustrial purposes and, naturally, increase the cost of the system andmay introduce factors which affect the reliability of magnesium as ahydrogen storage metal.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide acomposition for the storage of hydrogen by the reversible formation ofmagnesium hydride which eliminates the disadvantages of earlier systemsand which is capable of absorbing hydrogen rapidly without excessivetemperatures and pressures, which can have a relatively high capacityfor the hydrogen in the form of the hydride and which is capable of ahigh degree of desorption or regeneration of the hydrogen.

Another object of the invention is to provide a magnesium-basedcomposition for the purposes described which can be transformed in majorpart to the hydrogen-storing hydride and which maintains its highhydrogen capacity for a large number of hydrogen absorption/regenerationcycles.

It is also an object of this invention to provide an improved method ofmaking a magnesium-based composition for the storage and release ofhydrogen.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention, with a compositionfor the storage and regeneration of hydrogen which comprises an intimatepulverulent mixture of magnesium powder and a separating agenthomogeneously distributed throughout the composition and intermixed withthe magnesium powder, in the form of an inert second powder which can bepresent in an amount of 1 to 20% by volume of the magnesium powder.

The powder forming the separating agent is inert with respect tomagnesium, hydrogen and magnesium hydride in the sense that it does notcombine with magnesium or magnesium hydride and does not interfere withthe transformation of the magnesium powder to the hydride or thedecomposition of the hydride to magnesium and hydrogen under theconditions utilized for the formation of and the decomposition ofmagnesium hydride.

The separating agent powder is physically stable and inhibits theagglomeration of the magnesium powder so that the same retains itsoriginal subdivided state and specific surface area for numeroussuccessive absorption/desorption cycles for the storage and regenerationof gaseous H₂.

According to another aspect of the invention, the composition is formedby conjointly grinding or milling magnesium powder with the separatingagent in the form of a powder which is inert to hydrogen and magnesiumand is physically stable under the temperature and pressure conditionsnecessary for the formation and decomposition of magnesium hydridethereby forming the pulverulent intimate mixture in which the separatingagent is distributed uniformly in the composition and in an intermingledrelationship with the magnesium powder.

Naturally, the invention also relates to the product made by thisprocess and to a method of storing which comprises forming thepulverulent mixture described above, subjecting the resulting mixture tohydrogen contact with gaseous hydrogen at a pressure and temperaturesufficient to transform substantially all of the magnesium of themixture to magnesium hydride, retaining the mixture in which themagnesium has been transformed predominantly into the magnesium hydrideunder storage for an appropriate time, and subjecting the mixturethereafter to temperature and pressure conditions whereby the magnesiumhydride is thermally decomposed and gaseous hydrogen is releasedtherefrom. Generally speaking the process from contact of the mixturewith hydrogen to regeneration of the hydrogen will be repeated over amultiplicity of successive storage and regeneration cycles.

It has been found that a relatively large number of materials can beused as separating agents in accordance with the invention and with thedefinition given above, these materials being generally incapable offorming hydrides with hydrogen under the conditions necessary for theformation of MgH₂ and being nonreactive with the magnesium under thesame conditions. These materials include:

as elemental metals--manganese, iron, cobalt, nickel and copper;

as metal alloys--iron-zinc alloys and alloys containing one or more ofthe above-mentioned elemental metals and one or more of these metalsapart from iron in alloying relationship with the iron-zinc alloy; and

as compounds--the oxides, carbides or nitrides of magnesium, calcium,boron, aluminum, silicon, titanium, zirconium, chromium, manganese andiron.

Naturally, a combination of two or more such separating agents may beused as well and, for any particular industrial application, one canchoose from among the above-given class separating agents which do notinterfere with the particular industrial process to which the hydrogenis to be transferred or in which the composition is utilized.

Since the separating additive does not participate in the storage ofhydrogen it is advantageous to keep the amount of it in the compositionas low as possible so that the proportion of the composition constitutedby the convertible magnesium is as high as possible. Thus the minimumquantity of the separating powder which will prevent agglomeration andreduction and the surface activity of the magnesium should be used.Experience has shown that the separating agent should be present in anamount of 1 to 20% by volume of the magnesium powder so that themagnesium powder constitutes the preponderant component in thecomposition.

While the mechanism of the present invention is not certain, experiencehas shown that when magnesium particulates alone (without catalyst orseparating agent) are obtained in a relatively hard state, for examplefrom the machining of magnesium billets, it is possible to hydrogenatethem at 400° C. and 30 bar in about 15 hours for complete transformationto MgH₂. The quantity of hydrogen absorbed corresponds to about 7.4% (byweight of the magnesium). In order to desorb the hydrogen and recoverthe latter in a gaseous state from the magnesium, one subjects themagnesium hydride to thermal decomposition at a temperature in excess of300° C.

The thermal decomposition of the MgH₂ appears to reduce the degree ofhardness of the magnesium and bring about a certain degree ofinter-particle fusion (fritting or sintering) which considerably reducesfor subsequent cycles the storage capacity, e.g. 3 to 4% (by weight ofthe magnesium) or about half the initial capacity of about 7.4%resulting from the initial hydrogenation of the magnesium agent.

However, when the pulverulent agent of the present invention is added tothe magnesium, this fritting or sintering is eliminated and, when thecomposition is prepared by grinding the two powders together, it ispossible to obtain the extremely intimate mixture required to precludeloss of surface activity and at the same time to obtain a highlysubdivided state and hardness of the magnesium to facilitate the initialhydrogenation and transformation to MgH₂. Since the separating agent isused in small quantities and is essentially inert to the hydrogen itdoes not materially reduce the storage capacity of the composition whilemaintaining the storage capacity of the magnesium component whichotherwise would be lost in the manner described. Both hydrogenation anddehydrogenation occur with their original velocities, unimpeded byagglomeration of the magnesium particles. The composition has thus beenfound to allow, probably for the first time, the effective large-scaleuse of magnesium powder with exceptional efficiency in the storage andregeneration of gaseous hydrogen over a large number of cycles withoutsignificant loss in efficiency.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription and specific examples, reference being made to theaccompanying drawing in which FIGS. 1 through 6 are graphs in which themolar ratio MgH₂ /Mg is plotted along the ordinate and time is plottedalong the abscissa for systems corresponding to Examples 1 through 6infra.

SPECIFIC DESCRIPTION AND EXAMPLES EXAMPLE 1

An intimate pulverulent mixture of magnesium and iron (hereinafterreferred to as the mixture Mg-Fe) is prepared by grinding 25 g ofmagnesium powder in the form of fine magnesium turnings (marketed underthe name "FLUKA" 63040) with a particle size range between 200 and 50microns concurrently with 2.5 g of iron powder having a mean particlesize of 10 microns. The simultaneous grinding is carried out for aperiod of 72 hours in a ball mill using balls of a hard metal to producea very uniform pulverulent Mg-Fe mixture.

This mixture is subjected to activation in an autoclave adapted tosustain the temperature and pressure conditions for absorption anddesorption of hydrogen. The mixture is thus subjected to degassing for aperiod of 30 minutes at 350° C. under a primary vacuum of 10⁻² torr.

The absorption phase is effected by introducing hydrogen at a pressureof 30 bar into the autoclave which is maintained at a temperature of350° C. to 400° C. and in 15 hours hydrogenation is complete withsubstantially stoichiometric conversion of the magnesium to MgH₂. Aprecise weighing of the contents of the autoclave shows that thehydrogen pickup corresponds to a molar conversion of MgH₂ /Mg of 0.974.

The dehydrogenation or desorption phase is effected by reducing thepressure from 30 bar (during storage) to 1 bar and maintaining thetemperature at 350° C. until the MgH₂ is completely decomposed. The MgH₂is completely converted to magnesium (with evolution of H₂) in about 60minutes. When the temperature is raised to 400° C., the desorption timecan be reduced to about 30 minutes.

After this activation of the magnesium, samples of the mixture weresubjected to three series of tests, each involving 10absorption/desorption cycles in the manner described above and theconversion ratio as a function of the time for absorption of hydrogenwas plotted for each system after the tenth cycle. During the threeseries of tests, the Mg-Fe mixture was subjected to pressures of 30 bar,20 bar and 10 bar during the respective absorption (temperature of 350°C.-400° C.) with desorption at 350° C. and 1 bar. The three series oftests are represented on the graphs by Mg-Fe 30, Mg-Fe 20 and Mg-Fe 10,respectively, the figures 30, 20 and 10 representing for this exampleand those which follow the absorption pressures.

As a basis for comparison, a fourth series of tests was carried out withan equivalent quantity of the same magnesium powder, without addition ofiron, i.e. with 27.5 g of magnesium powder identical to that utilized inthe mixture and subjected to the same type of activation as was themixture. The results of this fourth series of tests are identified at Mg40 on the graphs.

FIG. 1 shows the experimental curves corresponding to the fourth seriesof tests in broken lines, and in solid lines the results obtained withthe mixtures.

As is clear from the curve Mg 30, the magnesium powder alone after 10cycles has a conversion ratio MgH₂ /Mg of 0.3 after 30 minutes and 0.4in 60 minutes for absorption of hydrogen. The storage capacitycorresponds respectively to 2.3 and 3.04% hydrogen (by weight ofmagnesium) which can be contrasted with the 7.4% initial value discussedpreviously.

The curve Mg-Fe 30 indicates a far more rapid and complete absorption ofhydrogen such that the conversion ratio MgH₂ /Mg has a value of 0.6 in30 minutes and 0.65 in 60 minutes (after 10 cycles), the storagecapacity after one hour at 30 bar being 4.5% hydrogen absorbed (byweight of the Mg-Fe mixture).

The Mg-Fe 10 curve demonstrates that a reduced hydrogenation pressure of10 bar during the absorption permits higher conversion ratios to beobtained, corresponding to 0.65 in 30 minutes and 0.73 in 60 minutes.The storage capacity after an hour at 10 bar is about equal to 5%hydrogen absorbed (by weight of the mixture Mg-Fe).

EXAMPLE 2

A pulverulent mixture of magnesium and cobalt (Mg-Co) was prepared andtreated in the manner described in Example 1 by grinding 25 g of thesame magnesium powder with 2.5 g cobalt powder having a mean particlesize of 100 microns.

After activation in the manner described in Example 1, the mixture Mg-Cowas subjected to three series of tests, each involving 10absorption/desorption cycles in which absorption of hydrogen was alsoeffected at 350° to 400° C., the pressure of hydrogen on absorptionbeing maintained at 30 bar, 20 bar and 10 bar, respectively, during thethree series of cycles. The ratio of conversion MgH₂ /Mg was in additiondetermined periodically for an hour of absorption in each cycle.

FIG. 2 illustrates the three experimental curves corresponding to Mg-Co30, Mg-Co 20 and Mg-Co 10, as well as the curve 30 already described inExample 1.

According to curve Mg-Co 30, the conversion ratio MgH₂ /Mg is 0.70 after30 minutes and 0.78 after 60 minutes. The storage capacity after onehour of absorption under 30 bar corresponds to 5.4% hydrogen (by weightof the mixture Mg-Co) after 10 cycles absorption/desorption, which isabout twice the corresponding capacity shown by the curve Mg 30 for themagnesium powder alone.

Absorption of hydrogen can be effected under a pressure ranging from 30to 10 bar without noticeably affecting the storage capacity of themixture Mg-Co, thus allowing effective utilization of this mixture at arelatively low pressure.

Another series of comparative tests has in addition shown thatabsorption of hydrogen under 30 bar but at a temperature of 300° C.allowed the same mixture Mg-Co to attain in 20 minutes the conversionratio MgH₂ /Mg of 0.76 corresponding to a storage capacity of 5.2%hydrogen (by weight of the mixture Mg-Co).

Consequently, the mixture Mg-Co can be used to absorb hydrogen at atemperature of only 300° C. and/or under the relatively low pressure of10 bar while ensuring the advantages described above regarding the morerapid and complete absorption of hydrogen in magnesium.

EXAMPLE 3

A pulverulent mixture of magnesium and nickel (Mg-Ni) was prepared andtreated in the manner described in Example 1 by grinding 25 g of themagnesium powder described in that example with 2.5 g nickel having amean particle size of 5 microns.

After activation carried out as described in Example 1, the mixture wassubjected to a series of tests; the results of these tests are shown bycurve Mg-Ni 30 in FIG. 3 indicating that:

The mixture Mg-Ni reaches the conversion ratio MgH₂ /Mg of about 0.5 in2 minutes and 0.55 in 10 minutes and then shows little increase during acourse of 1 hour.

The mixture of powder Mg-Ni can thus attain in 10 minutes the storagecapacity of 4% hydrogen (by weight of mixture Mg-Ni) and this storagecapacity remains constant at least during 10 subsequentabsorption/desorption cycles.

By comparison, according to the curve Mg 30, the magnesium powder alone(without any additives) absorbs hydrogen much more slowly and in asmaller quantity, e.g. 3% hydrogen (by weight of magnesium) in 1 hour ascompared to 4% hydrogen (by weight of the pulverulent mixture Mg-Ni) in10 minutes.

The mixture Mg-Ni obtained in accordance wih the present invention cantherefore absorb hydrogen more rapidly than the mixture Mg-Fe as shownin Example 1 but less completely than the mixture Mg-Co as shown inExample 2.

EXAMPLE 4

A pulverulent mixture of magnesium and copper (Mg-Cu) was prepared andtreated in the manner described in Example 1 by grinding 25 g of themagnesium as described with 2.5 g of copper powder having a meanparticle size of 25 microns.

The four curves in FIG. 4 illustrate the following:

Curve Mg-Cu 30 represents the molar conversion MgH₂ /Mg of about 0.5 in4 minutes, 0.75 in 30 minutes and 0.78 in 60 minutes.

The storage capacity attained in 1 hour of hydrogen absorption (under 30bar) by the mixture Mg-Cu corresponds to 5.3% and otherwise remainsconstant in at least 10 successive absorption/desorption cycles.

Hydrogen absorption is thus more rapid and complete in the mixture Mg-Cuthan in the magnesium powder alone (curve 30, FIG. 4).

As is clear from the curves Mg-Cu 20 and Mg-Cu 30, a lowering of thepressure to 20 and 10 bar corresponds as well to a more rapid andcomplete absorption of hydrogen in this mixture than in the magnesiumpowder alone under 30 bar (curve Mg 30, FIG. 4).

In addition, the above mixture Mg-Cu was subjected to tests of similarcycles, in which absorption was effected under 30 bar but withtemperature reduced to 300° C. These tests allow reversible storage of5% hydrogen (by weight of the mixture Mg-Cu), i.e. the storage capacitylowered very little, from 5.3% to 5%.

The mixture Mg-Cu can thus be used for absorption of hydrogen undernoticeably reduced pressure, within 30 and 10 bar (or even less), and/orat a temperature between 350° C. and 300° C. while allowing a more rapidand more complete absorption than is the case of magnesium alone.

EXAMPLE 5

A pulverulent mixture of magnesium, iron and zinc (Mg-ZnFe) was preparedand treated in the manner described in Example 1 by grinding 25 g of themagnesium as described in that example and 2.5 g of the powder of anequiatomic Zn-Fe alloy, having the composition Zn-Fe and a mean particlesize of 50 microns.

The mixture of Mg-ZnFe was subjected to an activation as described inExample 1, then to repeated absorptions (under 30 bar, at a temperatureof 350° C.) in a series of 10 absorption/desorption cycles.

FIG. 5 shows the corresponding curve Mg-ZnFe 30 and the curve Mg 30already described from which the following can be seen:

It is clear from curve Mg-ZnFe 30 that the molar conversion ratio MgH₂/Mg reaches 0.5 in 30 minutes and 0.66 in 60 minutes.

The storage capacity attained in 1 hour of hydrogen absorption (under 30bar, at a temperature of 350° C.) by the mixture Mg-ZnFe correspondsthus to 5% and remains otherwise constant in at least 10 successiveabsorption/desorption cycles.

Hydrogen absorption is thus more rapid and complete in the case of themixture Mg-ZnFe than in the case of magnesium alone (curve 30, FIG. 5).

EXAMPLE 6

A pulverulent mixture of magnesium and silicon carbide (Mg-SiC) wasprepared and treated in the manner described in Example 1 by grinding 25g of magnesium as described in that example with 2.5 g of siliconcarbide powder having a mean micron size of 200.

After carrying out the activation as described in Example 1, the mixtureMg-SiC is subjected to two series of 10 absorption/desorption cycles inwhich absorption of hydrogen is effected at 350° to 400° C., thepressure of hydrogen during the absorption was maintained at 50 bar inone series of cycles and at 30 bar in the other series, and the ratioMgH₂ /Mg was determined in the course of 1 hour of absorption.

FIG. 6 shows the respective experimental curves Mg-SiC 50 and Mg-SiC 30,as well as the curve Mg 30 already described. The following is clearfrom the curves:

According to curve Mg-SiC 30 the ratio MgH₂ /Mg rises to 0.58 in 30minutes and to 0.61 in 1 hour of absorption (under 30 bar, at 350° C.).

The storage capacity which is thus reached in 1 hour corresponds to 4.2%of hydrogen (by weight of the mixture Mg-SiC) and remains otherwiseconstant in the course of 10 cycles of this series under 30 bar.

The results represented by the curve Mg-SiC 50 show that absorption ofhydrogen is even more rapid and more complete at 50 bar than under 30bar (curve Mg-SiC 30).

The absorption of hydrogen effected during these cycles on the mixtureMg-SiC is thus much more rapid and complete than in the case ofmagnesium alone.

I claim:
 1. A composition for the storage of hydrogen which consists ofa pulverulent intimate mixture of magnesium powder and between 1 and 20%by volume thereof of a separator-agent powder inert to hydrogen andmagnesium under hydrogen absorption and desorption conditions, saidseparator-agent powder being uniformly distributed in said mixture,being physically stable and inhibiting agglomeration of the magnesiumpowder over a succession of absorption/desorption cycles whereby themagnesium powder is converted to magnesium hydride and hydrogen isreleased upon thermal decomposition of the hydride respectively.
 2. Thecomposition defined in claim 1 wherein said separator-agent powder iscomposed of at least one of the metals selected from the groupconsisting of manganese, iron, cobalt, nickel and copper or an alloythereof inert to hydrogen and magnesium under hydrogen absorption anddesorption conditions.
 3. The composition defined in claim 2 wherein theseparator-agent powder is a primary alloy inert to hydrogen andmagnesium under hydrogen absorption and desorption conditions.
 4. Thecomposition defined in claim 1 wherein said separator-agent powder iscomposed of at least one oxide, carbide or nitride of magnesium,calcium, boron, aluminum, silicon, titanium, zirconium, vanadium,chromium, manganese, iron or a combination thereof.
 5. A process forproducing a magnesium-based composition adapted to absorb hydrogen underpredetermined absorption conditions and form magnesium hydride andadapted, upon thermal decomposition of said magnesium hydride, torelease gaseous hydrogen, said process consisting of the steps ofconjointly grinding magnesium powder and a powder of a separator agentinert to hydrogen and magnesium, physically stable under said conditionsand adapted to prevent agglomeration of the magnesium powder duringabsorption/desorption cycling of the composition.
 6. The process definedin claim 5 wherein said separator-agent powder is composed of at leastone of the metals selected from the group which consists of manganese,iron, cobalt, nickel and copper or an alloy thereof inert to hydrogenand magnesium under hydrogen absorption and desorption conditions. 7.The process defined in claim 6 wherein said separator-agent powder is aprimary alloy of one of said metals inert to hydrogen and magnesiumunder hydrogen absorption and desorption conditions.
 8. A compositionfor the storage of hydrogen which consists of a pulverulent intimatemixture of magnesium powder and between 1 and 20% by volume thereof of aprimary iron-zinc alloy separator-agent powder inert to hydrogen andmagnesium under hydrogen absorption and desorption conditions, saidseparator-agent powder being uniformly distributed in said mixture,being physically stable and inhibiting agglomeration of the magnesiumpowder over a succession of absorption/desorption cycles whereby themagnesium powder is converted to magnesium hybride and hydrogen isreleased upon thermal decomposition of the hydride respectively.
 9. Aprocess for producing a magnesium-based composition adapted to absorbhydrogen under predetermined absorption conditions and form magnesiumhydride and adapted, upon thermal decomposition of said magnesiumhydride, to release gaseous hydrogen, said process comprising the stepsof conjointly grinding magnesium powder and a powder of a separatoragent in the form of a primary iron-zinc alloy.